The research proposed for this project is intended to augment our fundamental understanding of arsenic (As) behavior in ways that lead directly to a reduction in human exposure in both the U.S. and in Bangladesh. At the Vineland Chemical Superfund site in New Jersey, heavily contaminated with both inorganic As (InAs) and organic As (OrgAs), research will focus on reducing off-site transport of these contaminants in two ways. The effort will first be directed at polishing effluent from the existing treatment plant using a treatment column containing zero-valent iron (Fe(0)) filings which, this project's previous research has demonstrated, is capable of removing both InAs and OrgAs. Second, the feasibility of reducing off-site transport of As will be explored by installing and monitoring a permeable-reactive barrier containing Fe(0) filings. In Bangladesh, we are ethically compelled to lower the As exposure of study participants in Projects 2, 3, and 4. The proposed research will focus on the sustainability of continued withdrawals from those aquifers that are currently low in As. The justification is that these aquifers are currently the only realistic alternative for the approximately 50 million inhabitants of the country who have been drinking well water with an As content that often exceeds the WHO guideline of 10 ug/L by one to two orders of magnitude. Through detailed monitoring and targeted manipulations in the field and in the laboratory, the combination of hydrological, geochemical, and microbial processes that maintain As concentrations at low levels in both very shallow (<10 m) and deep (>30-150 m) aquifers will be investigated. In a direct application of the approach to mitigation developed under the previous round of funding, the exposure to As and Mn of children and adults, participating in Project 3 and 4 and residing in 25 villages, will be rapidly reduced by targeting safe aquifers for the installation of community wells. The timing of these interventions will be closely synchronized with parallel studies of their health impact, conducted under biomedical components of this application.